1. Field of the Invention
The invention relates to a rotary atomizer head, a rotary-atomizer painting device, and a rotary atomization painting method of electrostatic painting.
2. Description of the Related Art
A conventional rotary atomizer painting device is generally constructed such that a rotary atomizer head, equipped with an inner peripheral face that increases in diameter from the bottom thereof toward the tip thereof, is rotatably fitted to a painting device body and is rotated at high speed to apply a centrifugal force to a paint supplied to the bottom of the inner peripheral face to atomize and discharge the paint.
In the rotary-atomizer painting device, a high electrostatic voltage is applied to the rotary atomizer head to charge fine particles of the atomized paint. The charged particles of the paint are splattered toward a grounded object to be painted through an electrostatic electric field formed between the rotary atomizer head and the object to be painted. The surface of the object to be painted is thereby painted. As the rotary atomization painting device thus constructed, there is a painting device described in, for example, Japanese Utility Model Publication No. 6-12836 (JP-U-6-12836).
Further, as shown in FIGS. 9 and 10, a rotary atomizer head 101 that the described rotary atomization painting device is equipped with has an inner peripheral face 102 that increases in diameter from a bottom toward a tip thereof. The inner peripheral face 102 increases in diameter from the bottom 121 thereof toward the tip thereof (the near side of the sheet of FIG. 9, the left side of FIG. 10). Further, a paint discharge end 102c is formed at the tip of the inner peripheral face 102, and a dam portion 104 is formed on the inner peripheral face 102 between the bottom portion 121 and the paint discharge end 102c. 
The dam portion 104 is formed along the circumferential direction of the inner peripheral face 102, and is constructed as an annular member that extends from the inner peripheral face 102 substantially perpendicular to a rotary shaft. An opening 104b is provided in a central portion of the dam portion 104. Further, a region of the inner peripheral face 102 located between the bottom 121 and the dam portion 104 constitutes an inner paint channel 102a, and a region of the inner peripheral face 102 located between the tip and the dam portion 104 constitutes an outer paint channel 102b. 
Furthermore, a space surrounded by the dam portion 104 and the inner paint channel 102a constitutes a paint reservoir 122, in which a paint is held after being supplied from the bottom 121 and flowing to the tip side. Further, a plurality of paint supply holes 104a are formed in a circumferential direction of the inner peripheral face 102 along the boundary between the dam portion 104 and the inner peripheral face 102. The inner paint channel 102a and the outer paint channel 102b communicate with each other through the paint supply holes 104a. 
However, a communication hole 103 through which the bottom 121 and a base side of the rotary atomizer head 101 communicate with each other is formed through the bottom 121 of the inner peripheral face 102 of the rotary atomizer head 101 coaxially with the rotary shaft. A paint supply pipe 110 is inserted in the communication hole 103 from the base side of the rotary atomizer head 101. The paint supply pipe 110 has a closed tip. The tip of the paint supply pipe 110 protrudes from the bottom 121 of the inner peripheral face 102.
Further, a plurality of nozzle holes 110a are formed through a lateral face of the portion of the paint supply pipe 110 that protrudes from the bottom 121, which constitutes a paint supply nozzle 111.
When carrying out electrostatic painting with the rotary atomizer head 101 constructed as described above, the paint is supplied to the bottom 121 from the paint supply nozzle 111 when the rotary atomizer head 101 rotates at high speed. Then, the paint supplied to the bottom 121 flows toward the tip in the direction indicated by arrows A in FIG. 10 through the inner paint channel 102a, due to a centrifugal force created through rotation of the rotary atomizer head 101. The paint that has flowed from the bottom 121 to the tip through the inner paint channel 102a reaches the region in which the dam portion 104 is formed, where it is dammed by the dam portion 104, and is held in the paint reservoir 122.
The paint held in the paint reservoir 122 flows through the paint supply holes 104a out to the outer paint channel 102b in the direction indicated by arrows B, and is then atomized and discharged at the paint discharge end 102c of the inner peripheral face 102 in a direction indicated by arrows C.
As described above, in the rotary atomization painting device having the dam portion 104 formed on the inner peripheral face 102 of the rotary atomizer head 101, the paint reservoir 122, in which the paint is stored after being supplied to the bottom 121 and flowing to the tip side, is constructed.
Thus, if a large amount of paint is held in the paint reservoir 122 when the rotary atomizer painting device is turned OFF, it takes a long time to discharge all of the paint even after the rotary atomizer painting device has been turned OFF. More specifically, after the rotary atomizer painting device has been turned OFF, it takes several seconds for all the paint held in the paint reservoir 122 to be discharged. Therefore, the paint is not fluid enough. Further, an increase in operation time and a decrease in painting efficiency are caused due to the aforementioned construction. Moreover, if the painting device is turned off in an emergency, the paint in the paint reservoir 122 may drip, which may degrade the quality of the paint finish.